Cellular radio telephone systems now being deployed throughout the world generally consist of a mobile switching center (MSC) and a plurality of radio base stations (RBS), each RBS having a plurality of radio transceivers for servicing mobile units.
For every transceiver at the RBS there is typically a permanent and dedicated voice circuit between the RBS and the MSC. These circuits may be a facility leased from the local telephone company or other common carrier of wired communications, where the facility comprises dedicated lines. Alternatively, they may be privately owned communications facilities, which are also comprised of dedicated lines or dedicated microwave communications facilities. In any case, the circuits between the RBS and the MSC are permanent and dedicated regardless of the amount or frequency of use. Moreover, such an arrangement also requires the MSC to have a permanently dedicated port for each RBS transceiver in the cellular system.
The MSC also has a group of circuits over which calls to and from the local common carrier network are completed. This group of circuits is typically only large enough to handle the anticipated peak number of simultaneous calls and therefore there are fewer such circuits in the group than the sum of all transceivers in the system.
Since most systems consist of a plurality of RBSs, each with a plurality of transceivers, the number of transceivers in use at any point in time in comparison to the total number will vary from extremely low to moderately high. Averaged over an extended period of time, the percentage of usage for each of the MSC to RBS circuits is very low.
A typical cellular radio telephone system of the foregoing type is depicted in U.S. Pat. No. 4,698,839 which is herein incorporated by reference. These systems include a mobile switching center (MSC) connected to one or more radio base stations (RBS), each RBS containing a plurality of radio transceivers. The RBSs connect to the MSC through groups of dedicated telephone trunk circuits, which are provided by private facilities or leased from the common carrier network. In order for calls to be made between the cellular system and the common carrier network, the MSC further connects to the common carrier network by dedicated trunk circuits. The circuits between the MSC and the RBSs include both voice and data lines. The data lines are used to communicate control information between the MSC and the RBSs, and between the mobile phone and the MSC via the RBS. The voice lines are used to interconnect the voice circuit of the mobile telephone to the MSC, which switches the voice signals to the proper destination.
For a typical cellular radio telephone system, the following is an exemplary process flow of a call from a mobile telephone to a telephone in the common carrier network:
1. The mobile caller enters the telephone number that he or she wishes to be connected to, and directs that the call be made. PA1 2. The mobile telephone sends an origination request to the RBS by way of a radio frequency (RF) signal. PA1 3. The RBS decodes the RF signal as a control message and sends the message or its equivalent to the MSC over a data line. PA1 4. The MSC determines the validity of the mobile phone (i.e., whether the mobile telephone is registered to a system subscriber) and the validity of the request. PA1 5. If the mobile phone and request are recognized as valid, the MSC sends a message to the RBS to activate and appropriately tune a transceiver to provide service to the mobile telephone on a certain RF frequency and directs that a message be sent from the RBS to the mobile telephone, instructing it to use that certain RF frequency for the call. The MSC further assigns a dedicated voice line for the audio transmission between the MSC and the RBS. PA1 6. The mobile telephone tunes its transceiver to the designated frequency and, if appropriate, enables its audio circuitry. PA1 7. The mobile telephone now has a two-way audio path (over the RF carrier) to the RBS and over the dedicated voice circuit to the MSC. PA1 8. The MSC selects a trunk line from the group connecting the MSC to the common carrier network and signals the number to be called (e.g., a landline subscriber) to the network.
With the call in progress, the MSC has control of the call and can modify the switched connection between the MSC and the RBS during the call. For example, the MSC can handoff the call to another RBS or support features such as three-way calling by the mobile telephone.
Although the prior art systems as identified by the foregoing example work well, they inefficiently use trunk circuits, especially those dedicated as voice circuits between the MSC and the RBS, thereby increasing the cost of operating the system. This is because dedicated voice circuits connect the MSC to the RBS at all times, despite the amount and frequency of their actual use.
Such inefficiency is especially costly in microcell applications. Microcells, well known in the art, are small RBS cells. A typical microcell application is to serve a small area, within a macrocell (i.e. a larger RBS cell), that has a high demand for mobile service for short or intermittent periods of time. Examples might include an office complex that has a high demand during business hours and a commuter station that has a high demand during rush hours on business days. Other examples such as golf courses, beaches and other entertainment areas have a high demand during non-business hours.